docs: def get_top_edge_depth(self):
          """
          Return minimum depth of surface's top edge.
  
          :returns:
              Float value, the vertical distance between the earth surface
              and the shallowest point in surface's top edge in km.
          """
          top_edge = self.mesh[0:1]
          if top_edge.depths is None:
              return 0
          else:
              return numpy.min(top_edge.depths)